The present invention relates to a back-up battery management system for use with a primary DC power supply for telephone switching equipment or other loads, such as communication and computer equipment. In many other applications, the need for an uninterrupted source of DC power is critical.
To avoid any interruption or outage in power service, it is common practice to employ a battery back-up for the primary DC source. Back-up battery systems typically include strings of batteries or cells connected in parallel with the primary DC source and the load. In the event of a drop in the load bus voltage below a predetermined threshold, the back-up battery supplants or supplements the primary source of DC power. Back-up battery systems are designed to replace the primary DC power source for a predetermined period of time within which resumption of primary power is expected to occur.
In conventional back-up battery systems, the nominal system load bus voltage has typically been dictated by battery characteristics. For example, in a telephone switching plant, back-up batteries are commonly employed which have a design cell voltage of 2.25 volts for optimum health of the battery cell. Twenty-four cells are typically combined in a string resulting in a nominal load bus voltage of approximately xe2x88x9254 volts. A bank of strings supplies the necessary back-up DC power.
As the back-up batteries are placed across the load, the full 54 volts of system DC voltage are placed across the battery string. This design architecture of a typical back-up battery system presents a number of potential problems. Certain batteries, due to their electrochemical constitution, will draw more current than other batteries. All batteries, as they age, will experience increasing internal resistance and will draw more charging current from the main DC supply.
About a decade ago, a new type of lead acid battery was introduced into the marketplace. The battery is sealed, and allegedly requires no maintenance. In this type of battery, oxygen and hydrogen produced during electrochemical reactions in the battery recombine to maintain an aqueous liquid electrolyte at a constant level within the cell. As a result, these batteries have only a small amount of liquid electrolyte. These batteries have become known as xe2x80x9cvalve regulatedxe2x80x9d or xe2x80x9crecombinantxe2x80x9d or xe2x80x9celectrolyte-starvedxe2x80x9d batteries.
This type of lead acid battery (hereinafter termed xe2x80x9cvalve regulated lead acidxe2x80x9d or xe2x80x9cVRLAxe2x80x9d batteries) has often failed well before their design life, which is typically 10 years.
A particular battery may, for various reasons not clearly understood, begin to take on more amperage to maintain its charge. The increased charging current will elevate the temperature of the battery. The chemical recombination of the oxygen and hydrogen gases also creates heat. As the internal battery temperature increases, the current demand increases disproportionately. For every 10 degrees centigrade of increase in the battery""s internal temperature, the battery demand for current doubles. A battery in this condition will have one of two failure modes, the most damaging being xe2x80x9cthermal runaway.xe2x80x9d Thermal runaway may lead to an explosion of the battery, with likely destruction or severe damage to any nearby equipment. Alternatively, the battery may experience a xe2x80x9cmelt downxe2x80x9d and produce noxious gases that also are apt to damage or destroy neighboring equipment. The rectified AC source provided in typical telephone switching plants has more than ample capacity to supply any one or more batteries demanding abnormal charging current. This, together with paralleled battery strings, encourages the previously described thermal runaway or meltdown failures.
With the advent of fiber optic signal distribution, switching equipment has been decentralized, introducing a need for DC power supplies in unattended satellite installations distributed throughout the territory served. In these unattended installations, the equipment is often closely packed, leading to hostile thermal operating conditions for the equipment and increased occurrences of thermally induced failures. In less severe conditions, the placement of the back-up batteries directly across the load is apt to result in dry-out (loss of electrolyte), positive grid corrosion, and other problems which may lead to premature battery failure and/or below normal power performance.
Back-up battery systems must be monitored to determine the health and capacity of the batteries. The need to perform battery tests is particularly troublesome in systems that require the supply of an uninterrupted source of DC power. Testing of the vital statistics of a battery affecting output capacity, predicted life, etc. is presently done by taking a battery string off-line and testing it in one of two ways. The test procedure recommended by battery manufacturers as being the most reliable, is to discharge the battery into a load while measuring the response of the battery. The ability of a battery or battery string to hold a predetermined current level for a predetermined time is a reliable measure of the health and capacity of the battery. However, such discharge tests in the field require experienced personnel and are difficult and costly. Further, conventional battery testing, requiring the batteries to be taken off-line, suffers a loss of standby battery protection for the telephone plant or other equipment being supplied while the tested batteries are off-line.
To avoid the cost and inconvenience of a discharge test, it is commonplace to employ special field test equipment that tests for battery resistance, impedance, inductance and other parameters and characteristics without discharging the battery. See U.S. Pat. No. 5,250,904. However, as noted, tests that do not involve discharging the battery are apt to be less reliable.
U.S. Pat. No. 5,160,851 discloses a back-up battery system for telephone central office switching equipment. The back-up battery system includes that when the batteries are switched in circuit across the load, the cumulative battery voltage exceeds a predetermined load voltage for a selected period of time. A converter down converts the over voltage that results from switching extra cells across the load. The converter, a sensor for sensing the system discharge bus voltage and a switch may be formed as a single unit using MOSFET technology. One or more rechargeable batteries have cells floated at a given float voltage. A fail-safe contact switch may also be provided to parallel the MOS-FET switch and be operated in the event of MOS-FET failure.
Also, U.S. Pat. No. 5,777,454 discloses a back-up battery management system for use in a DC power supply system for use with telephone switching equipment or loads of other types. The disclosed battery management system is particularly adapted for use with batteries of the valve regulated lead acid type. It can also find utility with older xe2x80x9cfloodedxe2x80x9d lead acid type batteries and batteries of other types.
The disclosed invention also provided a back-up battery management system for a battery backed-up primary DC power supply that permitted the back-up batteries to be maintained on-line at all times. The back-up battery management system also included means for charging the batteries with a predetermined level of substantially constant current while isolating the batteries from the system load bus. The charging current is substantially constant at a given time and for a given condition of the battery.
The invention further disclosed that a control system is provided which monitors and controls all significant conditions and parameters within the back-up battery management system to maintain the battery system at a float charge during normal operation.
The present invention (ABC System) also concerns a back-up battery management system for use in a DC power supply for use with telephone switching equipment and loads of other types. However, the methods used to isolate the batteries from the load bus and administer different levels of charging current into the batteries differ substantially from the previously disclosed inventions by the inventor. These differences will become apparent when the circuitry is described and the claims are put forth.